1. Technical Field
The present invention relates to spark plugs for internal combustion engines.
2. Description of the Related Art
As ignition means in internal combustion engines, such as engines of motor vehicles, there are used spark plugs which have a spark gap formed between a center electrode and a ground electrode that are axially opposed to each other. Those spark plugs discharge a spark across the spark gap, thereby igniting an air-fuel mixture in a combustion chamber.
In the combustion chamber, them is formed a flow of the air-fuel mixture, such as a swirl flow or tumble flow. With the flow of the air-fuel mixture moderately flowing also in the spark gap, it is possible to ensure the ignition capability of the spark plug (i.e., the capability of the spark plug to ignite the air-fuel mixture).
However, depending on the mounting posture (or mounting state) of the spark plug to the internal combustion engine, part of the ground electrode, which is joined to a distal end of a housing of the spark plug, may be located upstream of the spark gap with respect to the flow of the air-fuel mixture. In this case, the flow of the air-fuel mixture in the combustion chamber may be blocked by the ground electrode, thereby being stagnated in the vicinity of the spark gap. As a result, the ignition capability of the spark plug may be lowered. That is, the ignition capability of the spark plug may vary depending on the mounting posture of the spark plug to the internal combustion engine. In particular, in lean-burn internal combustion engines which have been widely used in recent years, the combustion stability may be lowered depending on the mounting posture of the spark plug.
However, it is generally difficult to control the mounting posture of a spark plug to an internal combustion engine, i.e., difficult to control the circumferential position of the ground electrode of the spark plug relative to the internal combustion engine. This is because the mounting posture of the spark plug to the internal combustion engine varies depending on the state of formation of a male-threaded portion in the housing of the spark plug and the degree of fastening the male-threaded portion into a female-threaded bore formed in the engine.
To solve the above problem, Japanese Patent Application Publication No. JPH09148045A discloses two techniques for preventing the flow of the air-fuel mixture from being blocked by the ground electrode. The first technique is to form a slot-like hole in the ground electrode. The second technique is to fix the ground electrode to the housing through a plurality of thin plate-shaped members.
However, in the case of applying the first technique, the strength of the ground electrode may be lowered due to the formation of the slot-like hole in the ground electrode. Moreover, if the ground electrode was formed to have a large thickness for ensuring the strength thereof, it would become easier for the ground electrode to impede the flow of the air-fuel mixture in the combustion chamber.
On the other hand, in the case of applying the second technique, the shape of the ground electrode is complicated, thus increasing the manufacturing cost and lowering the productivity.